Polyesters, in particular, polyethylene terephthalate (PET) having excellent mechanical and chemical properties have been widely applied not only to clothes and industrial fibers, but also to magnetic tapes, photographic films, condenser films and molded products such as bottles.
Recently, it has been required to impart flame resistance to synthetic fibers and various plastic products from the viewpoint of the prevention of the fire. There has been an urgent need, in particular, to establish a process for producing flame resistant PET which is widely available in, for example, clothes, carpets, car seats and industrial films.
There have been proposed various processes for imparting flame resistance to polyesters. Among these processes, it is believed effective to add a phosphorus compound to a polyester. For example, U.S. Pat. No. 4,157,436 has proposed a process for producing a polyester for flame resistant fibers or films which comprises adding a specific phosphorus compound as a flame resistance-imparting component, which would deteriorate neither the properties of the molded product nor the workability, to a polyester and copolymerizing said phosphorus compound therewith.
From an economic viewpoint, however, this process is accompanied by some problems to be solved. Namely, in order to produce a polyester having a high degree of polymerization by this process, it is required to preliminarily convert the phosphorus compound to be added into a compound having a divalent esterifying functional group, which results in an increase in the production cost of the phosphorus compound or makes the production extremely difficult in some cases.
Further, Japanese Patent Publication No. 9178/81 has proposed a process for blending a phosphorus compound of a specific structure with a polyester. However, this process is also accompanied by some disadvantages from a practical viewpoint. That is to say, a phosphorus compound having a functional group which might depolymerize a polyester, such as a P-H bond, might significantly lower the degree of polymerization of the polyester or block the terminal group of the same, to thereby deteriorate the properties of the obtained polyester.